Decomposing any Sliceable into the first element and the rest in Swift.

Decompose.swift

/// Split the `Sliceable` collection `coll` into its head and tail.
/// Returns `nil` if the collection is empty.
func decompose<S : Sliceable>(coll: S)
    -> (S.Generator.Element, S.SubSlice)?
{
    let start: S.Index = coll.startIndex
    let end: S.Index = coll.endIndex
    return start == end ? nil
                        : (coll[start], coll[start.successor()..<end])
}

/// Split the `Sliceable` bidirectional collection `coll` into its last
/// element and everything before. Returns `nil` if the collection is empty.
func rdecompose<S : Sliceable where S.Index : BidirectionalIndexType>(coll: S)
    -> (S.Generator.Element, S.SubSlice)?
{
    let start: S.Index = coll.startIndex
    let end: S.Index = coll.endIndex
    if start == end {
        return nil
    } else {
        let last = end.predecessor()
        return (coll[last], coll[start..<last])
    }
}

// MARK: Examples
decompose([1,2,3])      //=> .Some(1, [2,3])
decompose([2,3])        //=> .Some(2, [3])
decompose([3])          //=> .Some(3, [])
decompose(Array<Int>()) //=> nil
decompose("foobar")     //=> .Some(("f", "oobar"))
rdecompose("foobar")    //=> .Some(("r", "fooba"))

This is a follow-up to http://www.objc.io/snippets/1.html.